Relax the SAMType#isInstantiable check

Narrowing the type is not necessary to check for instantiability, and it
prevents valid SAM types with wildcards from being used. This commit
does not contain any positive testcase because SAM types with wildcards
need another fix present in the next commit to work.

Author: smarter

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3811,9 +3811,8 @@ object Types {
     }
     def isInstantiatable(tp: Type)(implicit ctx: Context): Boolean = zeroParamClass(tp) match {
       case cinfo: ClassInfo =>
-        val tref = tp.narrow
-        val selfType = cinfo.selfType.asSeenFrom(tref, cinfo.cls)
-        tref <:< selfType
+        val selfType = cinfo.selfType.asSeenFrom(tp, cinfo.cls)
+        tp <:< selfType
       case _ =>
         false
     }

tests/neg/i2732.scala

@@ -0,0 +1,13 @@
+class B {
+  def f = 1
+}
+
+trait A { self: B =>
+  def g1(x: Int): Int
+  def g2 = g1(f)
+}
+
+object Test {
+  // A should not be a valid SAM type because it's not instantiable
+  val x: A = (y: Int) => y + y // error
+}